Mini Kobo freezes when unplugged

Hello everyone,

I received a kobo mini, but i'm facing a problem. When the device it's plugged in, it works fine, but when it's unplugged it doens't work. The screen freezes and I'm not able to do even a restar.

I've tried to reset it, even a hard one, but it doens't helped. I've downgraded the sfotware, and again doesn't help. I'm not reaching a solution to my problem.. Does anyone have an idea?

Many thanks, DM